A diesel particulate filter (hereinafter referred to as “DPF”), for example, is known as a filter for collecting particulate matter (hereinafter referred to as “PM”) in an exhaust gas emitted from a diesel engine.
The DPF has a limitation on an amount of collecting PM. Thus, it is necessary to carry out forced regeneration that periodically burns and removes the accumulated PM. The forced regeneration is performed by supplying unburned fuel, primarily hydrocarbon (HC), to an oxidation catalyst in an upstream exhaust passage through in-pipe injection or post injection, and raising the exhaust gas temperature to PM burning temperature with the heat generated upon oxidation.
Also, a selective catalytic reduction (hereinafter referred to as “SCR”) catalyst is known as a nitrogen compound (hereinafter referred to as “NOx”) catalyst for purifying NOx in an exhaust gas. The SCR catalyst selectively reduces and purifies NOx in the exhaust gas by using ammonia (NH3) produced from a urea solution (urea water) upon hydrolysis with heat of the exhaust gas.